I enjoy the creative aspect of this game more then the adventure at the moment. I was thinking that there should be an option to click on a block and move your character away and it will delete/build all the blocks within the square. So basically if you wanted to get rid of a 50x50 area of dirt, just click on one corner and go/fly to the far corner and WAALAA! its gone, and do the same with building.
When your in the destroying mode it makes all the blocks you want to destroy a see through red, and the blocks you want to make would be a see through green. In the bottom right corner It would display the size of what your destroying/building. Something that probably would look like this "43x67".